\begin{tikzpicture}[
    node distance = 2cm and 2cm,
    N/.style = {circle, draw, font=\Large, minimum size=1.1em},
    every edge/.style = {draw, -> , semithick, shorten >=2pt},
    every edge quotes/.style = {auto=right, inner sep=2pt}
    ]
    \node[N] (1) at (0,0) {1};
    \node[N] (2) at ([xshift=1.7cm, yshift=1cm]1) [label=right:{-}] {2};
    \node[N] (3) at ([xshift=1.7cm, yshift=-2.5em]1) [label=right:{-}] {3};

    
    \path (1) edge [->] node [above] {a} (2)
        (1) edge [->] node [below] {b} (3)
        (2) edge [loop above] node [above] {a} (2) %循环
        (2) edge [bend right] node [left] {b} (3)
        (3) edge [bend right] node [right] {a} (2);
    
\end{tikzpicture}